UNIVERSITY  OF   CALIFORNIA   PUBLICATIONS 


COLLEGE  OF  AGRICULTURE 

AGRICULTURAL  EXPERIMENT  STATION 


BERKELEY,  CALIFORNIA 


THE  CITRICOLA  SCALE 


BY 

H.  J.  QUAYLE 


BULLETIN  No.  255 

Berkeley,  Cal.,  May,  1915 


UNIVERSITY  OF  CALIFORNIA  PRESS 

BERKELEY 

1915 


Benjamin  Ide  Wheeler,  President  of  the  University. 
EXPERIMENT  STATION  STAFF 

HEADS   OF  DIVISIONS 

Thomas  Forsyth  Hunt,  Director. 

Eugene  W.  Hilgard,  Agricultural  Chemistry  (Emeritus). 

Edward  J.  Wickson,  Horticulture. 

Herbert  J.  Webber,  Director  Citrus  Experiment  Station ;  Plant  Breeding. 

Hubert  E.  Van  Norman,  Vice-Director;  Dairy  Management. 

William  A.  Setchell,  Botany. 

Myer  E.  Jaffa,  Nutrition. 

Robert  H.  Loughridge,  Soil  Chemistry  and  Physics  (Emeritus). 

Charles  W.  Woodworth,  Entomology. 

Ralph  E.  Smith,  Plant  Pathology. 

J.  Eliot  Coit,  Citriculture. 

John  W.  Gilmore,  Agronomy. 

Charles  F.  Shaw,  Soil  Technology. 

John  W.  Gregg,  Landscape  Gardening  and  Floriculture. 

Frederic  T.  Bioletti,  Viticulture  and  Enology. 

Warren  T.  Clarke,  Agricultural  Extension. 

John  S.  Burd,  Agricultural  Chemistry. 

Charles  B.  Lipman,  Soil  Chemistry  and  Bacteriology. 

Clarence  M.  Haring,  Veterinary  Science  and  Bacteriology. 

Ernest  B.  Babcock,  Genetics. 

Gordon  H.  True,  Animal  Husbandry. 

James  T.  Barrett,  Plant  Pathology. 

Fritz  W.  Woll,  Animal  Nutrition. 

A.  V.  Stubenrauch,  Pomology. 

Walter  Mulford,  Forestry. 

W.  P.  Kelley,  Agricultural  Chemistry. 

H.  J.  Quayle,  Entomology. 

William  G.  Hummell,  Agricultural  Education. 

Leon  M.  Davis,  Dairy  Industry. 

John  E.  Dougherty,  Poultry  Husbandry. 

Frank  Adams,  Irrigation  Practice. 

David  N.  Morgan,  Assistant  to  the  Director. 

Mrs.  D.  L.  Bunnell,  Librarian. 

CITRUS  EXPERIMENT  STATION 

Division  of  Entomology 
H.  J.  Quayle  C.  P.  Clausen 


THE  CITRICOLA  SCALE* 

By  H.  J.  QUAYLE 


INJUEY 


The  citricola  scale  is  one  of  the  most  injurious  scales  on  citrus  trees 
in  California.  It  not  only  necessitates  the  washing  of  the  fruit  because 
of  the  sooty  mold  fungus  but  it  seriously  impairs  the  vigor  of  the  tree 
and  consequently  greatly  reduces  fruit  production.  Some  groves  have 
been  seen  during  the  past  year  where  the  crop  was  reduced  as  much 
as  50  or  75  per  cent  because  of  the  attacks  of  this  scale.  In  a  certain 
grove  where  a  few  trees  were  treated  the  previous  year,  and  the  scale 
fairly  well  controlled,  these  trees  bore  considerable  fruit,  while  there 
was  practically  no  crop  on  the  untreated  trees  surrounding  them  which 
had,  aside  from  lack  of  scale  control,  the  same  care.  In  case  of  severe 
attacks  by  this  scale  there  is  therefore  not  only  a  very  evident  shortage 
of  the  crop  but  such  fruits  as  occur  are  deficient  in  sweetness  and  flavor. 

A  very  copious  amount  of  honeydew  is  given  off  by  this  scale, 
particularly  as  it  matures  in  the  spring  and  early  summer.  At  this 
time  it  is  produced  in  such  quantities  that  the  sooty  mold  fungus  is 
unable  to  utilize  it  all  as  a  medium  for  its  growth.  In  such  cases  the 
trees  have  an  oily  or  greasy  appearance,  and  not  infrequently  the 
honeydew  occurs  in  such  amounts  as  to  actually  drip  from  the  trees. 
When  the  honeydew  occurs  in  such  quantities  it  is  evident  that  there 
is  a  heavy  drain  upon  the  tree  through  the  loss  of  sap,  and  the  coating 
of  honeydew  over  the  foliage  (see  fig.  8)  interferes  with  the  normal 
functions  of  the  leaves.  The  same  material  on  the  fruit  also  necessi- 
tates the  washing  of  the  fruit  in  the  packing  house. 

IDENTITY  OF  THE  CITRICOLA  SCALE 

For  a  time  this  scale  was  confused  with  the  soft  brown  scale 
(Coccus  hesperidum),  which  it  resembles  very  closely.  That  this 
confusion  occurred  is  not  surprising  because  of  the  great  variation  in 
the  soft  brown  scale,  as  indicated  bv  the  fact  that  it  has  been  de- 


Note. — A  more  technical  paper  on  this  insect  is  in  preparation. 

*  Paper  No.  2,  Citrus  Experiment  Station,  College  of  Agriculture,  University 
of  California,  Riverside,  California. 

[405] 


406  UNIVERSITY   OF   CALIFORNIA EXPERIMENT  STATION 

scribed  many  different  times  under  different  names.  The  probability 
that  there  was  a  different  species  from  the  soft  brown  scale  in  the 
orange  groves  of  southern  California  was  first  suggested  by  certain 
factors  of  economic  bearing,  such  as  resistance  to  fumigation,  a  smaller 
number  of  parasites  attacking  it,  less  attractiveness  to  ants,  and  the 
more  general  distribution  of  the  scale  over  the  tree  and  grove. 

When  the  matter  of  identity  was  looked  into,  it  was  placed  under 
the  species  of  Coccus  longulus,  and  later  under  Coccus  elongatus,  as 
well  as  under  Coccus  hesperidum.  Certain  characters,  however,  did 
not  seem  to  agree  with  the  same  characters  in  any  of  these  scales,  and 
an  incidental  study  was  started  by  the  writer  in  1911.  During  a  part 
of  the  year  1913  Mr.  Roy  E.  Campbell  was  engaged  as  an  assistant 
to  devote  his  time  chiefly  to  this  insect.  As  a  result  of  Mr.  Campbell's 
studies  it  was  determined  as  a  new  species,  to  which  was  given  the 
name  Coccus  citricola.1 

General  Differences  in  Appearance  between  the  Citricola  and  the 
Soft  Brown  Scale. — While  the  citricola  scale  and  the  soft  brown  scale 
appear  similar  to  the  casual  observer,  there  are  certain  evident  dif- 
ferences upon  closer  examination.  The  citricola  scale  becomes  more 
distinctly  gray  in  color  as  it  approaches  maturity,  while  most  of  the 
soft  brown  scales  are  brown,  as  the  common  name  implies.  Some  of 
the  soft  brown  scales,  however,  approach  the  general  color  effect  of 
the  other  species,  but  usually  the  color  pigment  is  coalesced  in  definite 
areas,  instead  of  being  scattered  about  generally  in  a  peppery  effect, 
as  is  usually  the  case  with  citricola.  The  citricola  scale  is  also  more 
convex,  that  is,  more  rounded,  while  the  soft  brown  scale  is  generally 
very  flat.  The  young  of  the  citricola  scale,  however,  are  very  flat  and 
are  much  more  transparent  than  the  young  of  the  soft  brown  scale, 
which  may  take  on  the  brown  color  pigment  at  an  early  age. 

Differences  in  Distribution  Over  the  Tree. — In  its  distribution  on 
the  tree  the  soft  brown  scale  is  almost  invariably  limited  to  a  few 
twigs  or  branches.  This  scale  may  occur  in  great  numbers  over  this 
restricted  area,  but  the  rest  of  the  tree  may  be  entirely  free  from 
this  species.  On  the  other  hand,  the  citricola  scale  may  be  very  gen- 
erally distributed  over  the  entire  tree,  although  there  is  a  preference 
shown  for  the  north  side  and  usually  the  lower  half  of  the  tree.  This 
scale  as  it  matures  in  the  spring  and  early  summer  is  found  almost 
exclusively  on  the  twigs  and  smaller  branches.  Many  of  the  soft 
brown  scales  come  to  maturity  on  the  leaves.  The  citricola  scale,  also, 
occurs  largely  on  the  leaves  when  it  is  young,  but  almost  invariably 
migrates  back  to  the  twigs  to  complete  its  development. 


i  Entomological  News,  May,  1914. 


Bulletin  255  THE  CITRICOLA  SCALE  407 

Differences  in  Life  History. — There  are  several  generations  (three 
or  four)  of  the  soft  brown  scale,  while  there  is  but  one  generation  of 
the  citricola  scale,  or  at  most  a  partial  second.  The  citricola  scale 
always  deposits  eggs,  and  these  may  hatch  almost  immediately  or 
hatching  may  be  prolonged  for  two  or  three  days.  The  soft  brown 
scale  always  gives  birth  to  living  young.    . 

Other  Differences. — The  citricola  scale  does  not  possess  immunity 
from  the  attacks  of  parasites,  as  was  first  supposed,  but  parasites  attack 
this  scale  much  less  than  they  do  the  soft  brown  scale.  And.  finally, 
the  citricola  scale  after  reaching  a  certain  stage,  even  though  it  may 
still  be  very  small,  is  much  more  resistant  to  fumigation  as  ordinarily 
practiced. 

ORIGIN 

At  the  present  time  we  are  unable  to  account  for  the  origin  of  this 
scale.  It  seems  probable  that  it  is  an  introduced  species,  and  further 
studies  may  place  it  under  some  foreign  species  already  described. 


PRESENT  DISTRIBUTION  IN  CALIFORNIA 

The  citricola  scale  is  now  widely  distributed  over  the  state  (see 
fig.  1),  and  this  wide  distribution  would  seem  to  indicate  that  its 
occurrence  here  dates  considerably  further  back  than  its  first  recorded 
appearance.  It  was  first  found  in  the  vicinity  of  Claremont  in  1909 
and  at  about  the  same  time  near  Riverside  and  in  certain  sections  in 
San  Bernardino  County.  It  has  been  known  in  the  citrus  section  of 
Tulare  County  for  the  past  three  or  four  years,  although  its  occur- 
rence in  great  numbers  there  dates  back  but  a  year  or  two. 

In  the  counties  south  of  the  Tehachapi  the  citricola  scale  is  found 
in  Los  Angeles,  Orange,  Riverside,  and  San  Bernardino.  Thus  far  it 
has  not  been  reported  from  Santa  Barbara,  Ventura,  San  Diego,  or 
Imperial  counties.  In  San  Bernardino  County  infestations  occur  at 
Colton,  Recllands,  Highlands,  Rialto,  Etiwanda,  Cucamonga,  Upland, 
and  Ontario.  In  Riverside  County  the  infested  areas  are  in  the  vicin- 
ity of  the  city  of  Riverside  and  at  Highgrove.  In  Los  Angeles  County 
they  are  at  Claremont,  Pomona,  San  Dimas,  and  Glendora.  In  Orange 
County  one  infestation  is  found  near  Fullerton. 

In  the  counties  north  of  the  Tehachapi  the  most  general  infestation 
occurs  in  Tulare  County.  Practically  all  of  the  citrus  sections  north 
of  Piano  have  more  or  less  of  the  scale.  The  heaviest  infestations 
occur  around  Porterville,  Worth,  Globe,  Success,  Lindsay,  Exeter,  and 


408 


UNIVERSITY   OF   CALIFORNIA EXPERIMENT  STATION 


Orosi.  In  Fresno  County  the  citricola  scale  occurs  in  the  citrus  dis- 
tricts of  Mt.  Campbell  and  near  Sanger.  In  Sacramento  County  it 
occurs  in  the  Fairoaks  and  Orangevale  districts,  and  in  Placer  County 
at  Rocklin.  In  Yuba  and  Sutter  counties  it  has  been  reported  from 
citrus  trees  growing  in  the  cities  of  Marysville  and  Yuba  City.  In 
Contra  Costa  County  this  scale  has  been  reported  from  one  dooryard 
tree  in  Martinez.     In  Kern  County  one  tree  was  reported  infested 


Fig.   1. — Present   distribution   of   citricola   scale   in 
California  shown  by  X  on  the  map 


near  Bakersfield,  but  eradication  treatment  was  given  and  the  scale 
has  not  as  yet  reappeared. 

For  the  data  bearing  on  the  distribution  of  the  scale  by  counties 
as  given  above,  in  addition  to  his  own  inspections,  the  writer  is  in- 
debted to  the  respective  county  horticultural  commissioners.  It  should 
be  understood  that  the  localities  given  represent  the  distribution  at 
the  time  of  this  writing.  New  localities  are  continually  being  reported. 
This  is  partly  due  to  the  spread  of  the  citricola  scale,  but  also  because, 
in  many  cases  no  doubt,  previous  reports  referred  to  it  as  the  soft 
brown  scale. 


Bulletin  255 


THE  CITRICOLA  SCALE 


409 


Fig.  2. — Twig  of  orange  tree  showing  infestation  of  the  citricola  scale  as 
it  occurs  in  the  spring  and  early  summer 


Fig.  3. — Citricola  scale  on  orange  twigs,  enlarged 


410 


UNIVERSITY  OF   CALIFORNIA EXPERIMENT   STATION 


FOOD  PLANTS 

The  citricola  scale  is  primarily  a  pest  of  citrus  trees  in  California. 
It  has  been  found  on  a  few  other  plants,  but  in  all  cases  these  plants 
were  growing  in  the  vicinity  of  infested  citrus  trees.  These  plants 
include  the  nightshade,  pomegranate,  elm,  and  walnut.    The  fact  that 


Fig.  4. — The  soft  brown  scale,  Coccus  Inesyeridum,  on  branch  of  orange. 
This  species  is  the  only  close  relative  of  the  citricola  scale  in  the  orange 
groves  of  California  and  the  two  are  often  confused. 

this  scale  is,  at  present  at  least,  practically  limited  to  the  citrus  tree 
and  has  not  been  found  thus  far  on  any  native  plants  seems  to  indi- 
cate that  it  is  an  introduced  species. 


COMMON  NAME  OF  C.  CITRICOLA 

After  the  first  appearance  of  this  scale  it  was  commonly  known  as 
the  "longulus  scale,"  because  it  was  placed  under  the  species  longulus. 
It  has  also  been  called  the  "False  soft  brown  scale"  and  the  "Gray 


Bulletin  255 


THE  CITRICOLA  SCALE 


411 


scale."  Of  these  names  the  "Gray  scale"  may  be  most  appropriate, 
because  the  insect  is  of  a  grayish  color  when  mature,  and  there  is 
precedent  in  color  designation  since  most  of  our  other  citrus  scales 
are  so  named,  as  the  black,  red,  yellow,  and  purple.  While  Coccus 
citricola  is  of  a  grayish  color,  it  has  this  color  only  a  few  months  in 
the  year.  When  the  scale  is  small  during  the  summer  and  fall  it  is 
no  grayer  in  color  than  the  soft  brown  scale,  and  after  the  scale 
matures  and  dies  it  is  distinctly  brown  in  color.    For  this  reason,  and 


Fig.  5. — The  citricola  scale  as  it  appears  on  the  under  side  of  the  orange 
leaves  in  midsummer  and  fall.  At  this  season  the  scales  are  small  and 
inconspicuous.     The  photograph  is  natural  size. 

because  of  the  fact  that  the  limit  in  color  names  has  practically  been 
reached  already  and  further  additions  may  be  confusing,  it  has  been 
thought  best  to  call  this  the  "Citricola  scale"  after  its  specific  name. 


LIFE  HISTORY  AND  HABITS 

The  Egg. — Eggs  are  invariably  laid  by  the  citricola  scale,  and  this 
is  one  of  the  important  differences  between  this  and  the  related  species, 
the  soft  brown  scale.  In  some  cases  upon  first  examination  it  may 
be  thought  that  eggs  do  not  occur,  since  upon  lifting  the  parent  young 
scales  appear  with  no  evidence  of  eggs.     But  in  such  cases  hatching 


412 


UNIVERSITY   OF   CALIFORNIA EXPERIMENT   STATION 


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Bulletin  255  THe  CITRICOLA  SCALE  413 

has  already  occurred,  and  not  infrequently  hatching  may  occur  almost 
immediately  upon  the  extrusion  of  the  egg.  In  other  cases  many  eggs 
will  be  found  beneath  the  scale  and  hatching  may  be  prolonged  to  two 
or  three  days. 

The  number  of  eggs  deposited  by  this  scale  will  probably  range 
between  1000  and  1500.  Actual  egg  records  have  been  made  up  to 
1205  from  a  single  scale.  The  1205  eggs  were  deposited  during  a 
period  of  thirty  days,  making  a  daily  average  of  40.  The  individual 
egg-laying  period  will  average  about  thirty  days,  while  the  maximum 
period  may  reach  six  weeks  or  possibly  two  months.  The  first  eggs 
may  be  found  in  the  latter  half  of  April  and  they  continue  to  appear 
until  August. 

The  triangular  white  objects  that  are  seen  upon  turning  over  the 
adult  scale  are  the  skins  of  the  eggs  that  are  left  after  the  young 
scales  hatch.  These  increase  in  numbers  until,  at  the  end  of  the  egg 
period,  a  great  mass  of  them  may  be  seen  beneath  the  scale.  It  is 
possible,  though  it  is  a  very  tedious  task,  to  count  these  egg  skins  after 
all  the  young  have  emerged  and  thus  accurately  determine  the  total 
number  of  eggs  deposited.  It  is  these  egg  skins,  still  adhering  to  the 
twigs  after  the  old  scales  are  removed,  that  give  the  characteristic 
white  or  mealy  appearance  to  such  twigs. 

The  Young. — Upon  hatching  from  the  egg  the  young  may  remain 
beneath  the  parent  scale  for  from  a  few  hours  to  a  day  or  two,  de- 
pending upon  the  temperature  and  the  rate  at  which  the  eggs  are 
being  deposited.  The  young  crawl  about  actively  for  a  day  or  two, 
after  which  they  settle  largely  upon  the  under  sides  of  the  leaves. 
In  cases  of  severe  infestations  a  great  many  also  establish  themselves 
on  the  upper  surface  of  the  leaves  and  a  few  more  on  the  more  tender 
twigs.  The  first  molt  occurs  approximately  one  month  after  settling 
and  the  second  and  last  molt  one  month  after  the  first.  The  cast 
skins  may  be  detected  as  tubular  or  cornucopia-like  objects  (see  fig.  7c) , 
usually  with  indication  of  the  legs  extending  out  at  right  angles. 
These  skins  are  not  unlike  a  bit  of  thread  or  lint  and  often  remain 
attached  to  the  posterior  end  of  the  scale.  Such  objects  may  be  first 
observed  on  the  leaves  during  the  latter  part  of  June  and  may  be  seen 
continuouslv  after  that  until  late  in  the  fall. 

The  first  young  appeared  during  the  past  year  (1914)  about  April 
20th.  They  were  seen  at  that  time  in  Tulare  County  and  also  in  Los 
Angeles  County.  Only  occasional  young  appear  as  early  as  that  date, 
and  it  may  be  said  that  May  1st  is  approximately  the  beginning  of 
the  appearance  of  young  in  any  numbers.  Young  continue  to  appear 
through  May,  June,  and  July. 


414  UNIVERSITY  OF   CALIFORNIA EXPERIMENT  STATION 

During  the  late  summer  and  early  fall  the  scales,  while  on  the 
leaves  (see  figs.  5  and  6),  are  very  flat  and  more  or  less  transparent. 
By  November  there  is  an  appreciable  change  in  size  and  most  of  them 
become  darker  in  color.  At  this  time  also  migration  to  the  twigs 
begins.  This  migration  from  leaf  to  twigs  continues  slowly  through 
the  winter,  but  the  great  majority  migrate  to  the  twigs  in  March  and 
early  April.  Their  final  resting  place  is  on  the  under  side  of  the  twigs 
and  smaller  branches  and,  to  some  extent,  on  the  leaves.  Branches 
more  than  a  half  inch  in  diameter  rarely  have  scales  on  them. 

During  the  colder  weather  of  winter  the  scales  grow  but  little, 
remaining  more  or  less  dormant  until  the  warm  weather  of  spring, 
when  they  grow  very  rapidly  and  produce  eggs  before  May  1st.-  It 
is  during  this  period  of  rapid  growth  in  the  spring  and  during  the 
egg-producing  period  that  so  much  of  the  honeydew  is  given  off. 

The  Adult. — One  criterion  that  an  insect  has  reached  the  adult 
stage  is  when  it  has  molted  the  last  time.  But,  while  the  last  molt  of 
the  citricola  scale  occurs  in  August  or  September,  it  does  not  reach 
full  sexual  maturity  until  the  following  May.  It  requires  therefore 
practically  one  year  for  this  scale  to  complete  its  life  cycle.  We  have 
had  a  few  scales  on  potted  orange  trees  at  Riverside  that  completed 
their  life  cycles  in  a  shorter  time.  These  were  liberated  on  May  27, 
1914,  and  produced  a  few  young  the  following  October.  These  scales 
were  maintained  in  small  cages  on  the  leaves.  Some  scales,  out  of 
season  with  the  great  majority,  have  been  observed  also  in  the  field. 
But  with  a  possible  few  exceptions  this  scale  has  a  more  uniform  life 
cycle  than  the  black  scale  and  may  be  said  to  require  a  full  year. 

The  general  color  effect  of  the  scale  is  dull  gray.  This  is  due  to 
a  ground  color  of  yellow  with  dark  gray  or  black  markings.  These 
darker  markings  are  rather  uniformly  distributed,  giving  the  surface 
a  peppery  effect.  In  the  case  of  the  soft  brown  scale  this  dark  pig- 
ment is  usually  coalesced  in  definite  areas,  giving  a  more  blotched 
effect.  In  the  former  the  ground  color  of  yellow  disappears  just 
before  the  margin  is  reached,  giving  way  to  a  dull  gray.  Around  the 
entire  insect,  back  somewhat  from  the  margin,  there  is  a  more  or  less 
distinct  yellow  circle,  due  to  the  absence  of  the  dark  gray  or  black 
markings. 

After  the  adult  has  deposited  its  quota  of  eggs,  numbering  about 
1500,  which  process  is  completed  in  July,  it  soon  dies.  After  death 
the  scale  turns  decidedly  brown  in  color  and  is  much  more  likely  to 
drop  off  the  twig  than  some  of  the  other  scales.  In  fact,  during  the 
fall,  the  indication  that  old  scales  have  been  present  is  usually  repre- 


Bulletin  255 


THE  CITRICOLA  SCALE 


415 


sented  by  white  areas  due  to  the  egg  skins  which  have  remained  after 
the  adult  has  dropped  off. 

The  adults  of  this  species  occur  on  the  small  twigs  almost  entirely 
(see  fig.  2)  and  are  usually  distributed  over  the  entire  tree,  with  the 
majority  occurring  on  the  north  side.  The  character  of  the  honey- 
dew  given  off  appears  to  be  different  from  that  of  the  black  and  soft 
brown  scales  in  that  it  is  more  sticky,  and  it  is  more  difficult  to  remove 
from  the  fruit  the  growth  of  sooty-mold  fungus  on  it.  That  the  honey- 
dew  of  this  species  differs  from  the  honeydew  of  the  soft  brown  scale 


'V.i 


'■'  '-' 


•  ,ft>^ 

"■i^*^'* 

w 

- 

r       4^1 

-f~:%\ 

B 

■ 

& 

w^  i 

-^H 

^V/:;\                ■■" 

K    - 

Jraj 

'■■'--'■■' 

-r 

'■  3BHH 

fL 

MM 

m 

' 

»9 

Fig.  7.— A.  Partly  grown  soft  brown  scales  on  orange  leaf.  B.  On  the 
left  the  citricola  scale  and  on  the  right  the  soft  brown  scale  of  the  same 
age.  These  two  scales  emerged  on  May  27,  1914,  and  were  photographed 
on  August  5,  1914.  Age,  2  months  10  days.  C.  The  citricola  scale  with 
the  second  cast  skin  still  attached.  This  scale  hatched  on  May  27  and 
molted  the  second  time  August  4,  1914. 

is  also  indicated  by  the  fact  that  ants  are  not  attracted  to  it  in  such 
large  numbers. 

Summary  on  Seasonal  History. — The  first  eggs  are  deposited  by 
the  citricola  scale  during  the  latter  part  of  April  and  egg  deposition 
continues  until  August.  The  first  molt  of  the  majority  of  the  scales 
occurs  about  July  1st  and  the  second  molt  about  August  1st.  From 
the  time  of  hatching  until  the  following  March  the  scales  are  largely 
on  the  leaves.  During  the  spring  they  occur  on  the  twigs  and  smaller 
branches.     The  development  requires  approximately  one  year. 


416  UNIVERSITY  OF   CALIFORNIA EXPERIMENT  STATION 

NATURAL  ENEMIES 

As  compared  with  the  soft  brown  scale,  the  citricola  scale  is  fairly 
free  from  the  attacks  of  parasites.  While  parasites  attacking  this 
scale  are  common,  they  never  occur  in  large  numbers.  Most  of  the 
parasites  attack  this  scale  while  they  are  still  small.  They  occur  most 
abundantly  therefore  in  the  summer  and  fall.  One  of  the  commonest 
parasites  of  C.  hesperidum,  Microterys  flavus,  has  not  been  secured 
thus  far  from  C.  citricola.  We  have  taken  Coccophagus  flavoscutellum 
from  the  citricola  scale,  while  this  species  was  not  taken  by  Mr.  Tim- 
berlake2  from  the  soft  brown  scale.  Aphycus  flavus,  or  the  species  that 
has  been  going  under  this  name,  has  been  taken  from  C.  citricola  as 
well  as  C.  hesperidum,  Coccophagus  lunulatus,  and  C.  lecanii  and  have 
also  been  reared  from  both  species. 

The  only  direct  evidence  we  have  of  ladybird  beetles  feeding  on 
the  citricola  scale  is  in  the  case  of  the  twice-stabbed  species  Chilocorus 
bivulnerus  in  Fresno  County.  It  is,  of  course,  probable  that  some  of 
the  common  species  of  Coccinellids  occurring  in  southern  California, 
particularly  Rhizobius  species,  also  feed  on  the  citricola  scale. 

CONTROL* 

Spraying. — Because  of  the  denseness  of  foliage  and  the  growing- 
period  of  the  citrus  tree,  it  is  poorly  adapted  for  spraying  as  a 
means  of  controlling  scale  insects.  It  is  not  particularly  difficult 
to  kill  the  young  citricola  scale  with  a  number  of  different  sprays, 
but  it  is  exceedingly  difficult  to  apply  the  spray  thoroughly  enough 
to  kill  all  the  insects,  or  even  a  sufficient  percentage  to  insure  clean 
trees  and  fruit  the  following  year.  It  is  for  this  reason  that  the  value 
of  spraying  falls  far  short  of  fumigation,  when  the  fumigation  can 
be  done  satisfactorily,  but  on  account  of  the  short  period  when  this 
scale  yields  readily  to  the  gas  treatment  it  was  thought  at  the  begin- 
ning of  last  season  that  a  spraying  programme  might  be  carried  on 
to  advantage.  Spraying,  therefore,  has  been  very  generally  tried  out 
in  the  citrus  groves  of  California  during  the  past  year  not  only  where 
the  citricola  scale  occurs  but  also  where  the  black  scale  occurs.  The 
most  modern  power  spray  machinery  has  been  used  in  the  application 
of  practically  all  sprays  that  gave  any  promise,  but  the  results  have 
not  been  generally  satisfactory. 

2  Preliminary  Report  on  the  Parasites  of  Coccus  hesperidum  in  California,  by 
P.  H.  Timberlake.    Jour.  Ec.  Ent.,  Vol.  6,  No.  3,  p.  293. 

*  For  more  details  concerning  spraying  and  fumigation  see  Cal.  Ex.  Sta. 
Dir.  129. 


Bulletin  255  THE  ciTRICOLA  SCALE  417 

In  addition  to  the  spray  failing  to  reach  a  sufficient  number  of 
the  scales,  there  is  the  additional  objection  that  most  sprays  may,  at 
least  under  certain  weather  conditions,  do  injury  to  the  tree  and  fruit. 
Practically  all  of  the  sprays  tried  during  the  past  year  were  open  to 
this  objection  in  one  place  or  another.  When  the  spray  is  applied 
during  hot,  dry  weather,  or  during  "northers,"  or  "Santa  Anas,' 
the  so-called  electrical  periods,  burning  is  especially  likely  to  occur. 
This  is  particularly  true  in  the  case  of  the  oil  sprays.  Where  the 
soap  sprays  are  applied,  injury  is  just  as  likely  to  occur  during  damp 
or  foggy  weather.  The  soap  collects  in  drops  on  the  under  side  of 
the  fruit  and  slowness  of  evaporation  may  hold  the  soap  long  enough 
to  cause  burning.  Such  burned  spots  are  more  likely  to  occur  on 
fruit  in  the  interior  or  shady  part  of  the  tree.  The  oil  burning,  on 
the  other  hand,  usually  occurs  as  spots  on  the  upper  side  of  the 
fruit  in  the  direct  light. 

There  are  three  classes  of  sprays  that  include  most  of  the  materials 
that  have  been  used  for  the  control  of  the  citricola  scale.  These 
are  the  oil  sprays,  the  soap  sprays,  and  the  combination  of  oil  and 
soap. 

The  oil  sprays  include  the  kerosene  and  distillate  oils.  The  dis- 
tillate that  is  most  used  is  the  tree-distillate  of  31  to  32  degrees 
Beaume,  a  distillate  especially  refined  for  tree  use  and  put  out  by 
the  Southern  Refining  Company  of  Los  Angeles.  The  cost  of  this 
oil  in  Los  Angeles  is  5  cents  per  gallon.  This  is  used  at  a  strength 
of  from  2  to  3  per  cent  and  is  mixed  with  the  water  mechanically 
by  agitation  in  the  spray  tank. 

This  spray  is  improved  by  the  addition  of  a  small  amount  of 
soap  which  adds  to  the  penetrating  and  spreading  power  of  the  spray. 
The  formula  for  the  distillate  and  soap  is  as  follows : 

Distillate  Emulsion 

Tree  distillate,  31°-32°  4  gals. 

Liquid  soap  %  gal. 

or 

Hard  soap  :  4  lbs. 

Water  200  gals. 

To  prepare  this  spray,  first  place  the  soap  (if  hard  soap  is  used, 
it  must  first  be  dissolved  in  hot  water)  in  the  spray  tank  with  10 
or  15  gallons  of  water.  The  engine  is  now  started  and  the  emulsion 
is  made  by  the  materials  being  run  through  the  pump  under  pressure. 
After  a  few  moments  the  rest  of  the  water  may  be  added,  with  the 
pump  still  going,  when  the  spray  is  ready  to  be  applied  to  the  tree. 


418  UNIVERSITY   OF   CALIFORNIA — EXPERIMENT   STATION 

Under  favorable  weather  conditions,  particularly  in  the  coast  counties, 
this  spray  should  do  no  appreciable  injury  to  the  tree  or  fruit. 
During  hot  or  dry  weather,  however,  and  in  the  interior  sections, 
freedom  from  injury  cannot  be  assured. 

The  other  oil  spray  that  has  been  used  extensively  even  in  the 
interior  sections  without  injury  is  the  kerosene-water  mechanical 
mixture.  This  is  used  at  a  strength  of  8  to  10  per  cent  and  is 
mixed  with  water  mechanically  by  the  agitation  in  the  spray  tank. 
The  oil  used  in  this  case  has  the  trade  name  of  "Water  white"  or 
"W.  W. "  and  is  simply  a  cheap  grade  of  kerosene.  The  gravity  is 
42  degrees  Beaume  and  it  costs  7  cents  per  gallon  in  Los  Angeles. 

This  spray  may  be  improved  also,  in  the  opinion  of  the  writer, 
by  the  addition  of  soap  making  a  kerosene  emulsion.  The  formula 
for  this  spray  is  as  follows : 

Kerosene  Emulsion 

" Water  white"  or  "W  W"  oil,  42°  15  gals. 

Liquid  soap  %  gal. 

or 

Hard  soap 4  lbs. 

Water 200  gals. 


The  materials  used  are  mixed  in  the  same  way  as  explained  for  the 
distillate  emulsion. 

Of  all  of  the  sprays  that  have  been  applied  to  citrus  trees,  and 
many  have  been  tried  during  the  past  year,  the  kerosene,  or  kerosene 
emulsion,  has  proved  to  be  the  safest  from  the  standpoint  of  injury 
to  the  tree  or  fruit  and  at  the  same  time  effective  in  killing  the  scales. 
In  the  case  of  young  trees,  however,  where  there  is  a  considerable 
amount  of  the  spray  allowed  to  run  down  the  trunk  there  is  liable 
to  be  injury  just  below  the  surface  of  the  ground.  This  girdling 
of  the  small  tree-trunks  by  the  kerosene  seems  especially  likely  to 
occur  in  sandy  soils.  Where  small  trees  are  sprayed  and  much  of 
the  material  runs  down  the  trunk,  the  precaution  should  be  taken 
to  remove  three  or  four  inches  of  the  soil  immediatelv  around  the  tree 
by  means  of  a  hoe,  and  replace  the  soil  thus  removed  by  fresh  soil. 
This  should  be  done  immediately  after  making  the  spray  application, 
certainly  not  later  than  the  following  day.  The  distillate  Or  distil- 
late emulsion  is  much  less  likely  to  do  this  sort  of  injury  to  small 
trees  than  the  kerosene  spray,  probably  because  of  the  higher  con- 
centration of  the  kerosene  spray,  and  also  its  greater  penetrating 
action.     Small  trees  growing  in  sandy  soil,  and  bearing  fruit  which 


Bulletin  255  THE  CITRICOLA  SCALE  419 

might  not  otherwise  be  spotted,  may  be  most  satisfactorily  sprayed 
with  the  distillate.  The  important  consideration  in  spraying  young 
trees  is  to  wet  them  well  and  allow  no  more  spray  to  drip  on  the 
ground  or  run  down  the  tree  trunk  than  necessary.  Where  the 
ground  is  thoroughly  saturated,  the  bark  on  some  of  the  roots  that 
are  within  two  or  three  inches  of  the  surface  may  also  be  injured 
from  the  kerosene. 

Application  of  the  Spray. — From  the  discussion  in  the  proceeding 
pages  it  will  be  seen  that  several  different  sprays  may  kill  the  cit- 
ricola  scale,  and  also  the  black  scale,  while  they  are  still  quite  small. 
One  of  the  important  considerations  in  selecting  a  spray  for  citrus 
trees,  therefore,  is  one  that  will  do  the  least  injury  to  the  tree,  and 
probably  the  most  important  of  all  in  the  spraying  programme  is  the 
proper  application. 

Satisfactory  results  should  not  be  even  expected  unless  the  spray 
is  most  thoroughly  applied  to  the  tree.  Since  the  citricola  scale 
is  very  largely  on  the  under  side  of  the  leaves  at  the  usual  time  for 
spraying,  the  spray  should  be  directed  almost  entirely  from  below. 
Two  angle  nozzles  on  a  Y  at  the  end  of  each  rod  is  the  best  arrange- 
ment. These  nozzles  should  not  throw  too  coarse  a  spray,  otherwise 
too  much  material  will  be  used  if  any  attempt  at  thoroughness  in 
covering  all  parts  of  the  tree  is  made.  The  "Bean  Mist"  and  "Bean 
whirlpool"  nozzles  are  satisfactory,  or  the  "Friend  Drive  Spray' 
nozzles  when  discs  with  the  smaller  openings  are  used.  Two  applica- 
tions are  necessary;  usually  the  first  of  these  should  be  made  for 
the  citricola  scale  in  August  or  September,  and  the  second  a  few 
weeks  later. 

FUMIGATION 

The  most  satisfactory  treatment  for  the  control  of  the  citricola 
scale  is  fumigation.  But  unlike  the  black  scale  this  species  is  sus- 
ceptible to  the  gas  for  a  very  short  period  of  the  general  fumigation 
season  (July  15  to  January  1).  The  earliest  work  in  fumigating  for 
this  scale  where  close  observations  were  made  on  the  results  wras  done 
by  Mr.  D.  Kell,  inspector  for  the  Pomona  district.  This  work  showed 
that  the  best  results  were  secured  where  the  fumigation  was  done 
early.  That  early  fumigation  is  more  effective  has  proved  to  be  true 
also  from  the  writer's  observations  and  experiments  of  the  last  two 
or  three  years.  Since  in  Tulare  County  during  the  past  year  a  heavier 
schedule  was  employed  than  was  customary  in  Southern  California, 
it  was  thought  that  the  effective  fumigation  period  might  be  pro- 
longed.    But   the   experience   there   was   the   same   as   the   previous 


420  UNIVERSITY  OF  CALIFORNIA — EXPERIMENT  STATION 

experience  in  the  south,  namely,  that  after  about  the  middle  of 
September  effective  work  was  much  less  certain. 

Season. — The  fumigation  period  for  the  citricola  scale  is,  there- 
fore, between  July  15  and  September  15.  In  several  cases  satisfactory 
work  has  been  done  as  late  as  October  or  even  November,  but  at 
this  time  there  is  no  positive  assurance  that  satisfactory  results  will 
be  secured  after  the  middle  of  September.  We  cannot  say  why  this 
scale  becomes  so  much  more  resistant  than  others  at  so  early  a  date, 
on  the  basis  of  any  perceptible  change  occurring  in  the  insect  itself. 
It  is  true  that  the  scale  is  constantly  getting  older  and  possibly  because 
of  that  fact  more  resistant,  but  judging  from  the  size  and  appearance 
of  this  scale,  as  we  are  accustomed  to  judge  the  black  scale,  it  ought 
to  succumb  readily  to  fumigation  as  late  as  December  or  January. 
The  citricola  scale  has  almost  the  same  development  period  as  the 
black  scale.  It  grows  very  slowly  and  there  is  but  a  slight  difference 
in  size  during  the  first  three  or  four  months.  By  the  middle  of 
September  practically  all  of  the  citricola  scales  have  molted  the  second 
time,  and  it  is  possible  that  the  resistance  is  largely  acquired  after 
this  final  molt.  But  the  majority  have  passed  through  the  last  molt 
before  the  middle  of  September  and  thus,  on  such  a  basis,  poor  fumi- 
gation results  ought  to  be  expected  earlier. 

During  the  time  the  citricola  scale  is  most  susceptible  to  fumi- 
gation, between  July  15  and  September  15,  satisfactory  work  has 
often  been  done  with  the  75  per  cent  schedule,  but  we  have  seen 
scale  survive  this  dosage.  Where  it  is  possible  to  use  it,  without 
injury  to  the  trees  or  fruit,  a  100  per  cent  schedule  is  more  certain. 
In  the  coast  counties,  however,  it  is  not  safe  to  use  the  100  per  cent 
schedule  without  some  danger  of  doing  damage  to  the  trees.  In 
the  interior  sections,  as  at  Riverside,  the  full  schedule  may  be  used 
with  no  bad  results.  In  Tulare  County,  moreover,  a  110  per  cent 
schedule  has  been  generally  used  during  the  past  year  with  very  little 
injury  to  the  tree.  The  drier  air  and  the  escape  of  more  gas  must 
account  for  this  variation  in  the  different  localities.  It  was  suspected 
at  the  beginning  of  the  present  season  that  fumigation  would  not  be 
possible  in  the  San  Joaquin  Valley  before  September  on  account 
of  the  heat.  Experimental  fumigation  was  done  in  that  section, 
however,  as  early  as  the  middle  of  June.  But  at  this  season  the  fruit 
was  too  small  to  be  sufficiently  resistant  to  the  gas.  Commercial 
fumigation,  under  the  general  supervision  of  Mr.  R.  P.  Cundiff,  an 
experienced  fumigator  of  Southern  California,  was  however,  started 
by  the  middle  of  July  and  continued  until  well  into  the  fall.     In 


Bulletin  255  THE  citricola  SCALE  421 

July  and  August  fumigation  was  carried  on  up  to  a  temperature  of 
80  degrees.  It  must  be  stated,  however,  that  the  summer  of  1914  was 
cooler  than  the  average  in  Tulare  county,  and  future  seasons  may  be 
less  satisfactory  for  the  process. 


SUMMARY 

1.  The  citricola  scale  is  one  of  the  economically  important  citrus 
insects  of  California. 

2.  It  is  at  present  distributed  over  widely  separated  localities  and 
in  some  sections  is  spreading  rapidly. 

3.  The  young  appear  by  the  last  week  in  April  and  continue  to 
appear  until  August. 

4.  During  the  summer,  fall  and  winter  the  scales  are  found  on  the 
leaves  almost  exclusively  and  grow  very  slowly. 

5.  In  November,  and  later,  a  few  migrate  back  to  the  twigs  but 
the  greatest  migration  occurs  in  March. 

6.  With  the  warm  weather  of  spring  they  rapidly  mature  and 
begin  to  deposit  eggs  late  in  April. 

7.  One  full  year  is  thus  required,  usually,  for  the  life  cycle. 

8.  Fumigation  between  July  15  and  September  15  is  the  most 
satisfactory  treatment.  Fumigation  results  are  less  certain  later  in 
the  year. 

9.  Where  fumigation  is  not  feasible,  spraying,  as  explained  in  the 
text,  may  be  employed. 


STATION    PUBLICATIONS    AVAILABLE    FOR    DISTRIBUTION 


REPORTS 

1897.      Resistant  Vines,  their  Selection,  Adaptation,   and  Grafting.      Appendix  to  Viticultural 
Report  for  1896. 

1902.  Report  of  the  Agricultural  Experiment   Station  for   1898-1901. 

1903.  Report  of  the  Agricultural   Experiment   Station  for   1901—03. 

1904.  Twenty-second  Report  of  the  Agricultural  Experiment  Station  for  1903-04. 

1914.     Report  of  the  College  of  Agriculture  and  the  Agricultural   Experiment  Station,   July, 
1913-June,    1914. 

BULLETINS 


No. 
168. 

169. 
170. 
174. 
177. 

178. 
182. 

183. 
184. 

185. 

195. 
197. 


198. 
203. 

207. 
208. 
211. 


Observations   on    Some   Vine   Diseases 

in    Sonoma  County. 
Tolerance  of  the  Sugar  Beet  for  Alkali. 
Studies  in  Grasshopper  Control. 
A  New  Wine-Cooling  Machine. 
A    New    Method    of    Making   Dry    Red 

Wine. 
Mosquito  Control. 
Analysis    of    Paris    Green    and    Lead 

Arsenate.     Proposed  Insecticide  Law. 
The  California  Tussock-Moth. 
Report    of    the    Plant    Pathologist    to 

July  1,  1906. 
Report  of  Progress  in   Cereal  Investi- 
gations. 
The  California  Grape  Root-worm. 
Grape  Culture  in  California  ;  Improved 

Methods     of     Wine-making;      Yeast 

from  California  Grapes. 
The  Grape  Leaf-Hopper. 
Report    of    the    Plant    Pathologist    to 

July   1,    1909. 
The  Control  of  the  Argentine  Ant. 
The  Late  Blight  of  Celery. 
How  to  Increase  the  Yield   of  Wheat 

in  California. 


No. 
212. 
213. 
216. 


220. 
225. 
227. 
230. 
234. 
240. 
241. 
242. 
243. 

244. 
246. 
248. 

249. 
250. 
251. 


252. 


California  White  Wheats. 

The  Principles  of  Wine-making. 

A    Progress    Repoi-t    upon    Soil    and 

Climatic     Factors     Influencing    the 

Composition  of  Wheat. 
Dosage  Tables. 

Tolerance  of  Eucalyptus  for  Alkali. 
Grape  Vinegar. 
Enological  Investigations. 
Red  Spiders  and  Mites  of  Citrus  Trees. 
Commercial  Fertilizers. 
Vine  Pruning  in  California.     Part  I. 
Humus  in   California   Soils. 
The  Intradermal  Test  for  Tuberculosis 

in  Cattle  and  Hogs. 
Utilization  of  Waste  Oranges. 
Vine  Pruning  in  California.     Part  II. 
The  Economic  Value  of  Pacific  Coast 

Kelps. 
Stock  Poisoning  Plants  of  California. 
The  Loquat. 
The    Utilization    of    the    Nitrogen    and 

Organic  Matter  in  Septic  and  Imhoff 

Tank  Sludges. 
The  Deterioration  of  Lumber. 


CIRCULARS 


No. 
65. 
69. 

70. 

76. 
79. 
80. 
82. 

83. 
84. 
87. 

88. 


91. 
92. 

100. 
101. 

102. 
106. 

107. 


The  California   Insecticide  Law. 
The  Extermination  of  Morning-Glory. 
Observations   on   the    Status   of   Corn 

Growing  in  California. 
Hot  Room  Callusing. 
List  of  Insecticide  Dealers 
Boys'   and   Girls'   Clubs. 
The    Common    Ground     Squirrels    of 

California. 
Potato  Growing  Clubs. 
Mushrooms  and  Toadstools. 
Alfalfa. 
Advantages  to  the  Breeder  in  Testing 

his  Pure-bred  Cows  for  the  Register 

of  Merit. 
Disinfection  on  the  Farm. 
Infectious    Abortion    and    Sterility    in 

Cows. 
Pruning  Frosted  Citrus  Trees. 
Codling    Moth    Control    in    the    Sacra- 
mento Valley. 
The  Woolly  Aphis. 
Directions  for  using  Anti-Hog-Cholera 

Serum. 
Spraying  Walnut  Trees  for  Blight  and 

Aphis  Control. 


No. 
108. 
109. 


110. 
111. 

13. 
114. 
115. 
117. 

118. 
119. 
121. 

122. 

123. 

124. 
125. 
126. 
127. 
128. 


Grape  Juice. 

Community  or  Local  Extension  Work 
by  the  High  School  Agricultural  De- 
partment. 

Green  Manuring  in  California. 

The  Use  of  Lime  and  Gypsum  on  Cali- 
fornia Soils. 

Correspondence  Courses  in  Agriculture. 

Increasing  the  Duty  of  Water. 

Grafting  Vinifera  Vineyards. 

The  Selection  and  Cost  of  a  Small 
Pumping  Plant. 

The  County  Farm  Bureau. 

Winery  Directions. 

Some  Things  the  Prospective  Settler 
Should  Know. 

The  Management  of  Strawberry  Soils 
in  Pajaro  Valley. 

Fundamental  Principles  of  Co-opera- 
tion in  Agriculture. 

Alfalfa  Silage  for  Fattening  Steers. 

Aphids  on  Grain  and  Cantaloupes. 

Spraying  for  the   Grape  Leaf-Hopper. 

House  Fumigation. 

Insecticide  Formulas. 


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